Conventional magnetic coating thickness gauges employ constructions that make them complicated and therefore expensive to manufacture. Their design makes cost so prohibitively high that it is very rare to find an auto body shop, painter, or small manufacturing firm that owns one. Presently, the two lowest cost gauges commonly available are Tinsley at over $150 and Pentest (Reg. U.S. Pat. & TM. Off.) at nearly $200.
The Tinsley gauge is representative of prior art gauges that do not record the reading. Its structure is relatively complicated and expensive to manufacture. Its housing has two nonsymmetrical portions, is made of nonmagnetic metals, and resembles a thin pencil. The leading end of the housing is bored to slidably receive an interior magnetic member which is about the diameter of a pencil lead. This portion of the housing also has material removed from two opposite sides to form flat surfaces upon which the lines and numbers of the indicia are located. Longitudinally extending slots are formed in the center of both flat surfaces to expose the interior member to view. The trailing end portion of the housing has a bore of larger diameter to receive a spring.
The present invention, by comparison, has a single piece of thin-walled tubing as its housing. The housing is symmetrical with no slots, drilling, or machining required, and can be manufactured if desired with inexpensive materials such as extruded plastic.
In addition to substantial savings in manufacture, the present invention also has significant advantages in ease of reading. The Tinsley indicator must be viewed through a slot of less than 0.035 inch width, while this invention has an indicator typically more than ten times larger at 0.410 inch. Furthermore, the present invention permits larger numbers, numbers adjacent to the markings as opposed to being separated by a slot, and room for two or more additional adjacent sets of indicia plus additional room for add-on indicia tailored to the end user's needs. The present invention also uncovers the numerical readings on the indicia as the housing is being withdrawn from the surface being tested. When using a gauge that doesn't record readings, this is a significant aid to remembering the maximum reading.
The Pentest gauge, disclosed in U.S. Pat. No. 4,392,305, is representative of the least expensive gauges which record readings. Its construction is also relatively complicated and expensive to manufacture. It has over twelve components compared to the present invention's recording ratchet embodiment which has five.
Gauges which record readings by using a micrometer screw feed system of increasing tension on the spring, such as disclosed in U.K. Pat. No. 799,768, U.S. Pat. No. 2,625,585, Austria Pat. No. 215169, Soviet Pat. No. 590591, and U.K. Pat. No. 907721 are still more complicated and expensive to manufacture.
Devices such as the SNITCH Body Filler Detector and the device disclosed in U.S. Pat. No. 4,634,974 are not gauges, possessing no scales and unable to produce accurate comparative readings. Both devices are designed to detect whether or not the nonferrous coating is greater than a predetermined value. If the plunger of either device can be held at a maximum point of travel inside the housing, then the coating thickness is less than the predetermined one value. Even though these devices are not sophisticated enough to produce gauge readings, their housings are of irregular shape and therefore more expensive to manufacture than the tubular housings of the present invention. The magnet members are also more complicated on these devices, employing four components as distinguished from two on the present invention.
FIFI, also called Filler Finder, is another device which only provides a yes or no indication to its predetermined values. The display is accomplished with two lights.
Other devices such as disclosed in U.S. Pat. No. 4,433,290 are considerably different in construction and use. This referenced device is intended to compare surfaces and is constructed with a permanent magnetic rod and a ferromagnetic rod supported and held in parallel alignment by two nonmagnetic sliders.
Other prior art includes components of magnetic thickness gauges such as the pole piece described in U.S. Pat. No. 3,761,804 which relates specifically to magnetic micrometers. Calibration methods such as disclosed in U.S. Pat. No. 4,160,208 are of general interest but have no direct bearing on the present invention.